Ontogenetic approach to cellular localization of neurotransmitters in the chick vestibule

Gamma-aminobutyric acid (GABA) and acetylcholine (Ach) have been implicated in afferent and efferent neurotransmission, respectively, in the vestibular sensory periphery. Assuming that glutamate decarboxylase (GAD), the GABA-synthesizing enzyme, and choline acetyltransferase (ChAT), the enzyme for synthesis of acetylcholine, are located in distinct cell types of the inner ear whose maturation occurs at different times during ontogenesis, we measured these enzymes in the ampullary cristae of embryonic chicks at different stages of development. By making these measurements in parallel with electron-microscopic studies of the different cell elements of the chick vestibular sensory periphery, we found that the values of GAD activity were nearly the same from the earliest stage studied, i.e., the 13th day of ontogeny to day 18 of embryonic development, paralleling the morphologically mature appearance of the hair cells and their afferent synapses. A slight increase in enzymatic activity from day 19 of ontogeny to one day after hatching corresponded to a rise in the number of afferent synapses. In contrast, ChAT activity was practically undetectable up to day 17 of embryonic development, but rose suddenly on the 19th day, reaching 1-day-old levels by day 20 of ontogenesis in coincidence with an elevation in the number of well-developed efferent boutons. These results are in accord with the localization of GAD in the sensory cells and a localization of ChAT in the efferent nerve endings. These findings suggest that GABA and Ach are the respective neuromediators for the afferent and efferent systems.